Copending U.S. Pat. application Ser. No. 369,628 filed April 19, 1982, discloses an improved process for converting low performance ferromagnetic oxides to high performance ferromagnetic oxides. In that process a plurality of layers each containing a transition metal are deposited on the surface of the particles. One preferred embodiment includes three such layers in which the first and third layers contain cobalt and the intermediate second layer contains ferrous iron. The layers are precipitated in sequence onto the particles from an aqueous solution. As each layer is deposited the solution is heated for an extended period of time and then cooled before the second layer is deposited. The heating and cooling steps are then repeated and thereafter the third layer is deposited, heated and cooled.
The total sequence of steps in the referenced process consumes a substantial period of time. For example, using the three layer structure as typical, the first layer is heated for three hours at 100.degree. C and then cooled to room temperature. The slurry forming the second layer is heated to 100.degree. C. for sixteen hours and then cooled. Then the slurry containing the third layer materials is heated to 100.degree. C. for three hours and cooled. Thus, the heating time alone consumes about twenty-two hours.
While the prior process is effective and produces ferromagnetic particles having the desired properties, the process is disadvantageous from a practical commercial standpoint because of the time involved. The present invention is designed to provide comparable ferromagnetic particles having improved properties for magnetic recording, particularly with respect to increased coercivity, by depositing multiple layers containing transition metals but does so in a very much shortened period of time. The present process is thereby more useful from a commercial point of view.